1. Field of the Invention
This invention relates to nuclear reactors, and more particularly to neutron shield and reflector assemblies in liquid cooled fast reactors.
2. Description of the Prior Art
A typical fast breeder reactor includes three radially distributed zones in its core region. These are first and innermost, fuel assemblies containing fissionable nuclear material, second, radial blanket assemblies containing fertile nuclear material for breeding fissionable nuclear material, and third, and outermost, removable radial shield assemblies to protect core support structures and the reactor pressure vessel from excessive radiation damage and to reflect neutrons back into the core.
A coolant, such as a liquid metal or inert gas, is circulated through the core to remove heat generated by the fissioning process. The heat flux generally decreases progressively from the core center radially outward. Consequently, different cooling requirements and assembly structures have been utilized in the three zones. The fuel assemblies typically include a plurality of small diameter rods maintained in relative position by various types of grid or other spacing structures; the radial blanket is similar, with typically larger and fewer rods per assembly. This rodded arrangement provides the large cooling surface area necessary for the coolant flowing through these hotter assemblies, and physical separation of the rods. The removable radial shield assemblies, however, require significantly less cooling. Also, in order to properly perform their protective function, there is a premium placed upon a high density material distribution in the removable radial shield zone.
As a result of the high density and low cooling criteria, typically shield assemblies have been solid, with coolant flow passageways therethrough. These have taken the form, for example, of graphite or metallic blocks of hexagonal or circular cross-section with flow holes axially therethrough. Other shield assemblies have included transverse layers of metal tightly stacked about a central axial flow tube. Although such shielding assemblies adequately protect the surrounding support structures from excessive neutron bombardment, they are not without deficiencies.
First, because of the importance of density to perform a protective function, the solid shield assemblies present a large mass. As this dense mass radially surrounds the core, and typically may also be utilized above and below the core, it presents significant safety-oriented concerns relating to core response under seismic accident or other high loading conditions. To mitigate these concerns, complex design approaches and structures must be added to the reactor.
Second, because the shield assemblies have a high mass and a relatively small coolant flow, they are susceptible to considerable thermal stresses. The thermal coefficient of expansion acts along this large mass, inducing thermal stresses with changes in reactor power level. These stresses may also introduce undesirable bowing in the shield assembly region, and consequently apply further undesirable forces to the inner core assemblies. The thermal stresses further may reduce the life of the shield assemblies, requiring time consuming replacement at relatively close intervals.
Third, in order to mitigate the negative effects of the large mass and high thermal stress conditions, the material composition of the solid or axial stacked shield blocks is rather limited. The shortened life, coupled with the lack of material selection flexibility, may significantly increase the capital and operating costs of a fast reactor over its design lifetime.
Among the prior art structures which are limited by these deficiencies is that described in U.S. Pat. No. 3,219,540 in the name of D. Costes. It includes a nuclear reactor with the internal wall of the reactor vessel lined with canned and removable elements of solid graphite moderator material, which also functions as a neutron reflector. Because graphite is incompatible with liquid coolants such as sodium in a reactor environment, the elements must be clad with a suitable material, such as zirconium or magnesium. Such designs are not only costly to manufacture, but also enhance the concerns associated with thermal stresses due to differential expansion between the clad and the graphite. Also, the mass of such elements which extend substantially the length of the vessel presents seismic response limitations.
Another prior art structure is described in U.S. Pat. No. 3,549,493 in the name of J. H. Germer, which includes a reactor core made up of bundles which are closely packed together and held together by an orificing pattern. The bundles include hexagonal fuel, blanket, and reflector bundles, the latter surrounding the outer edge of the core. Each reflector bundle may be composed of solid stainless steel penetrated by cooling passages. This structure, therefore, presents an undesirable large mass surrounding the core, and a separate orificing or hydraulic balancing arrangement to support the assemblies. As a solid mass, however, the structure is also susceptible to thermal stresses and undesirable seismic loads.
While some fuel assemblies such as that described in U.S. Pat. No. 3,011,962 in the name of L. J. Koch et al contain fuel rods of fissionable nuclear material within a protective metal cladding, spaced within an enclosing duct, it is to be noted that this invention does not relate to fuel assemblies but is directed to neutron shield and reflector assemblies which typically surround the fuel assemblies. Not only are the functional requirements of these assembly types different, but also the limiting design criteria are very different. For example, because the fuel rods generate a high thermal and neutron flux, they must be provided with much higher coolant flows than required in the shield assemblies.
Also, in order to prevent undesirable boiling and bubble formation along the rods, typically both a high coolant flow rate and some device to induce swirling flow about the rods are utilized. Such devices may include wire wrapping about the rods, or flow deflectors, either in grid-structures which align the fuel rods or in a fuel rod duct. Merely a predominantly vertical flow along the fuel rods will cause boiling at a lower than desirable fuel rod temperature.
Furthermore, to similarly minimize "hot-spot" or wear locations along the fuel rods, they are spaced from one another. The structures used for this spacing are typically the grids or wire wrap referred to above. It is therefore to be noted that the design criteria and resulting structures applied to fuel assemblies are significantly different than those applied to shield assemblies.
It is therefore seen that significant operational, safety, and cost benefits can be obtained from a neutron shield assembly which alleviates the above deficiencies of the prior art.